Metal mask screen for screen-printing

ABSTRACT

A printing screen comprises a fine metal mesh stretched across a printing frame. A central printing area of the screen has a layer of metal integrally united with the mesh. This central metal layer has a pattern of openings therein corresponding to a pattern which is to be printed on a substrate and outwardly extending tabs at points of stress. An annular region of the mesh surrounding the central metal layer is free of any metal layer but has a thin masking layer of synthetic resin thereon. A border layer of metal on the mesh extends around the outer periphery of the screen.

United States Patent 1 Griffin Nov. 6, 1973 METAL MASK SCREEN'FOR FOREIGN PATENTS OR APPLICATIONS KEEN-PRINTING SC 664,713 1/1952 Great Britain 101/127 [75] Inventor: Patrick Joseph Griffin, Indianapolis,

Primary Examiner-Robert E. Pulfrey [73] Assignee: RCA Corporation, New York, NY. Asslstam Emmmer R' Suter [22] F! d A 28 1972 Att0rneyGlenn H. Bruestle et al.

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21 Appl. No.: 284,075 71 ABSTRACT I A printing screen comprises a fine metal mesh [52] U s 0 101/127 101/128 2 stretched across a printing frame. A central printing [51] Int B4ln l/2'4 area of the screen has a layer of metal integrally united [58' Field 2 128 with the mesh. This central metal layer has a pattern of 7/35 l7 openings therein corresponding to a pattern which is to be printed on a substrate and outwardly extending tabs [56] References Cited at points of stress. An annular region of the mesh surrounding the central metal layer is free of any metal UNITED STATES PATENTS layer but has a thin masking layer of synthetic resin 2,025,457 l2/l935 Kavanaugh 101/1282 thereon A border layer of metal on the mesh extends 32323; ig l /fi s around the outer periphery of the screen. 3,694,245 9/1972 Anderson et al 101/1282 UX 3 Claims, 2 Drawing Figures nlllllll" METAL MASK SCREEN FOR SCREEN-PRINTING BACKGROUND One type of screen commonly used in screen printing comprises a fine mesh of thread-like wires having a masking coating composed of a hardened photographic emulsion. This type of screen has the advantages of being highly flexible and conformable to surface irregularities on the substrate but has the disadvantage of not being sufficiently wear resistant for use in long printing runs where extreme accuracy of outline must be maintained throughout the entire run.

In theelectronics industry, increasing use is being made of thick-film microcircuits made by screenprinting a pattern of conductors and resistors, and sometimes other components such as capacitors, from inks made primarily of glass frits, metals, metal oxides and various vehicles, on ceramic substrates, and then firing. It is particularly important that the values of the resistors change as little as possible as a printing run progresses. However, with photo-emulsion type screens, resistor values do change to an excessive degree, requiring that compensations be frequentlymade during a printing run.

It has previously been known that printing screens which are highly-wear-resistant can be made by electrolytically depositing a layer of nickel on one side of a screen made of a metal such as phosphor bronze or stainless steel coated with nickel, and then etching a pattern of openings through the nickel layer using an etchant such as ferric chloride. The nickel layer welds the screen wires together at their crossover points.

This type of screen has previously been limited in its use mostly to fiat contact printing because of its relatively rigid nature. Although flexible, it is not flexible enough to be satisfatorily used with a squeegee ink applicator and off-contact printing. By off-contact is meant that the screen is, normally suspended a short distance above the substrate being printed and the inkapplying squeegee forces successive areas of the screen into contact with the substrate as the printing stroke progresses.

When use of a metal mask type printing screen, as described above, was attempted in an off-contact type process, cracks often developed in the metal mask within the printing area and the screen was rendered useless after a short run.v

SUMMARY OF THE INVENTION The present invention is an improved metal mask printing screen suitable for use in an off-contact type printing process. The screen comprises a metal fine mesh having adhered to one surface a central metal layer with a pattern of openings corresponding to the pattern to be printed. Surrounding the central metal layer is a mesh region free of anymetal layer but covered with a masking layer of synthetic resin. Extending into this area from the central metal layer are metal tabs which are located at stress points. The screen also includes a border metal layer on the mesh printing surface around the outer periphery.

THE DRAWING FIG. 1 is a plan view of an embodiment of a printing screen in accordance with the present invention, and

FIG. 2 is a section view taken along the line 2-2 of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT The basic material out of which the printing screen of the present invention is made in a fine metal mesh having a layer of metal foil integrally united to one of its surfaces. This is a commercially available product which is made by electrolytically depositing a layer of a metal such as nickel on one surface of a stainless steel mesh which has first been provided with a thin coat of nickel. The mesh can have any desired number of openings per square inch up to about 325. A 165 mesh screen is preferred for this embodiment. The nickel layer may be about 1 mil thick, for example. The surface of the mesh on which the metal layer is disposed becomes the printing surface when the printing screen is completed.

The completed printing screen ll, as illustrated in the drawing, has a central metal layer 2 on the printing area, with a pattern of openings 4 therein. The pattern of openings corresponds to the pattern which is to be printed on a substrate. In this example, the printed pattern represents a network of conductors which are part of an electronic circuit (not any specific circuit, however) which is to be deposited on a ceramic substrate. After this pattern is dried and fired, a second pattern will be printed, in register, depositing a number of resistors across gaps left in the'pattern of conductors.

Surrounding the central metal layer 2 is a region 6 of the mesh which is free of any metal layer but does have a plurality of metal tabs 8 extending into the region from the central metal layer 2. These tabs are an integral part of the central layer 2 and are located at stress points around the periphery of layer 2. The region 6 is coated with a masking layer 10 of synthetic resin.

Around the periphery of the screen is a border layer 12 of metal disposed on the mesh which gives added strength to the screen.

In setting up the screen for use in a printing operation, it must be stretched taut so that proper release of the screen will take place after the squeegee has passed, i.e., so that it will spring back and out of contact with the surface being printed after the contactmaking pressure of the squeegee is removed. To maintain tension, the screen is mounted in a metal frame 14. The frame 14 has a groove 16 around the periphery, into which the edges of the screen are forced and locked in place by locking strips I18. In use, the screen is mounted, printing face down. Ink is supplied to the back surface of the screen and a squeegee is swept across the back surface, in this example from left to right or right to left (or both). As the squeegee travels across the screen it forces portions of the screen into contact with the substrate and forces ink through the openings 4.

In making a printing screen, the entire nickel layer is coated with a photoresist. The photoresist may be any commercially available photoresist.

The photoresist is then exposed through a master pattern so that light strikes those parts of the resist layer where metal is to remain. The resist layer is then developed so that the parts of the metal layer which are to be removed, are uncovered. Etching of the nickel may be carried out with a solution of ferric chloride of 42 B. containing 1 percent by volume of concentrated hydrochloric acid. Etching time is about 1 10 seconds at F. The etching step is followed by a water rinse and then a drying step.

The remaining photoresist is then stripped off. This leaves the central metal layer 2 with the pattern of openings 4 and the metal tabs 8 around the periphery of the layer 2. It also leaves the region 6 which is free of any metal layer, surrounding the central metal layer 2 and a border layer of metal 12.

The region 6 is then provided with a masking layer 10. This may be done using a diazo-sensitized hydrolyzed polyvinyl alcohol emulsion. The entire surface is coated with emulsion and by photomasking and developing techniques, all unwanted emulsion is removed except the layer 10.

The metal-free region 6, as stated previously, is desirable to impart sufficient flexibility to the screen to be used for off-contact printing. The border layer 12 of metal is provided to add strength. In a screen having an area of about 8-54 inches by lO- r inches, a metal border layer 12, l-% inches wide has been found sufficient.

When the central metal layer 2 is used without metal tabs, parts of the metal layer became coined and indented. With metal tabs 8 placed at points where stresses are found to occur, the screen can be used for long printing runs. Stress usually occurs at the corners of a pattern and along the edges which are perpendicular to the line of travel of the squeegee. The tabs are placed at these locations. However, size and placement of the tabs will depend upon the sizeand outline of the central metal layer 2. a

A further precaution to prevent cracks and other imperfections from appearing around the edges of the central metal layer 2 is to leave a wide enough border of metal outside the area where the openings 4 are located. It has been found, in practice, that this border should be at least about Vs inch wide where the central printing layer has dimensions of about l-r inches by 2-5 inches.

I claim:

1. A metal mask screen for off-contact screenprinting comprising a fine metal mask adapted to be stretched on a printing frame,

a central printing area of said screen having a central layer of metal integrally united to one surface of said mesh,

said central layer of metal having a pattern of openings therein corresponding to a pattern which is to be printed on a substrate,

a region of said mesh surrounding said central layer of metal being free of any metal layer but having a thin, flexible masking layer of synthetic resin adhered thereto, and

metal tabs, which are a continuation of said central layer of metal, extending outwardly therefrom at points of stress and into said region which is free of any metal layer.

2. A screen according to claim 1 including a border layer of metal adhering to said mesh around the outer periphery thereof.

3. A screen according to claim 1 in which said metal mesh is composed of stainless steel coated with nickel and said central layer of metal is nickel. 

1. A metal mask screen for off-contact screen-printing comprising a fine metal mask adapted to be stretched on a printing frame, a central printing area of said screen having a central layer of metal integrally united to one surface of said mesh, said central layer of metal having a pattern of openings therein corresponding to a pattern which is to be printed on a substrate, a region of said mesh surrounding said central layer of metal being free of any metal layer but having a thin, flexible masking layer of synthetic resin adhered thereto, and metal tabs, which are a continuation of said central layer of metal, extending outwardly therefrom at points of stress and into said region which is free of any metal layer.
 2. A screen according to claim 1 including a border layer of metal adhering to said mesh around the outer periphery thereof.
 3. A screen according to claim 1 in which said metal mesh is composed of stainless steel coated with nickel and said central layer of metal is nickel. 